tests: bluetooth: mesh: Add separate assert macro with message support

ASSERT_OK macro is confusing as it accepts additional arguments, but
don't use them. This commit removes the confusion by making ASSERT_OK
macro accept only one argument. An additional macro ASSERT_OK_MSG is
added that mandates a format string that will be printed and optionally
arguments for the format string.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>